1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte.
2. Description of the Background Art
Non-aqueous electrolyte secondary batteries are commonly available today as secondary batteries having high energy density. In a non-aqueous electrolyte secondary battery, for example, charge and discharge occur by the transfer of lithium ions between a positive electrode and a negative electrode.
In such a non-aqueous electrolyte secondary battery, in general, a complex oxide of lithium transition metals having a layered structure of lithium nickelate (LiNiO2), lithium cobaltate (LiCoO2) or the like is used as the positive electrode, and a carbon material that can store and release lithium ions, a lithium metal, a lithium alloy, or the like is used as the negative electrode (refer to, for example, JP-2003-151549-A).
In addition, an organic solvent such as ethylene carbonate or diethyl carbonate in which an electrolyte salt such as lithium borate tetrafluoride (LiBF4) or lithium phosphate hexafluoride (LiPF6) is dissolved is used as the non-aqueous electrolyte.
While these non-aqueous electrolyte secondary batteries have recently been used as power sources for mobile equipment, a need exists for developing non-aqueous electrolyte secondary batteries having higher energy densities with increasing power consumption caused by expansion in functionality of the mobile equipment.
Li2MO3 crystal (M indicates a tetravalent transition metal) is known as a material having a similar layered structure to that of the above-mentioned LiCoO2. This Li2MO3 crystal is expected as a positive electrode material having a high capacity due to its high lithium content, and researches concerning the Li2MO3 crystal have been conducted by research institutes. A theoretical capacity as large as 459 mAh/g can be obtained in Li2MnO3 using low-priced manganese (Mn) as the above-mentioned tetravalent transition material.
While Li2MnO3 is expected as a positive electrode material having a high capacity, however, results of a charge/discharge test in the case of using Li2MnO3 for a positive electrode show the problems of low charge-discharge efficiency and capacity maintenance ratio (for example, refer to C. S. Johnson et al., Electrochem. Commun. 6, (2004) 1085). The above-mentioned charge-discharge efficiency (%) is defined by the ratio of specific discharge capacity to specific charge capacity and the above-mentioned capacity maintenance ratio (%) is defined by the ratio of the specific discharge capacity in a predetermined cycle (for example, tenth cycle) to that in a first cycle.
The above-mentioned problems are attributed to the fact that some of lithium ions released from a positive electrode containing Li2MnO3 in a charge process are not stored in the positive electrode in a discharge process. In addition, as charge-discharge cycles become longer, the quantity of lithium ions that are not stored increases. As a result, the charge-discharge efficiency and capacity maintenance ratio further decrease.